1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling the phase of a video signal in video tape recorders (VTR) which do not record a CTL signal on a magnetic tape or an editing system that employs such VTRs.
2. Description of the Related Art
One conventional editing system for selectively recording video signals from a plurality of video sources in a single video tape cassette is illustrated in FIG. 1 of the accompanying drawings. The editing system shown in FIG. 1 is of the simplest design among editing systems generally known as A/B roll editing systems.
The A/B roll editing system shown in FIG. 1 comprises a console 106 having a liquid-crystal display (LCD) unit for displaying a time code and various control keys including a mix/wipe key, a playback key, a record key, a stop key, a fast-feed key, a rewind key, an edit start key, an edit end key, and other keys, a controller 104 for controlling various circuits in the editing system based on control information entered through the console 106 and a reference signal supplied from an external source through an input terminal 105, and for supplying the reference signal to the various circuits in the editing system, a playback VTR 100 for playing back a video tape cassette as an A roll, a playback VTR 101 for playing back a video tape cassette as a B roll, a switcher 102 for switching between an A-roll video signal Va from the playback VTR 100 and a B-roll video signal Vb from the playback VTR 101 based on a control signal from the controller 104, and a recording VTR 103 for recording an edited A/B-roll effect video signal which is produced by switching between the A-roll video signal Va and the B-roll video signal Vb with the switcher 102, in a video tape cassette used as a master tape.
In FIG. 1, the solid-line arrows extending from the controller 104 to the playback VTRs 100, 101, the switcher 102, and the recording VTR 103 represent the control signal, and the broken-line arrows extending from the controller 104 to the playback VTRs 100, 101, the switcher 102, and the recording VTR 103 represent the reference signal.
Operation of the A/B roll editing system shown in FIG. 1 will be described below with reference to FIG. 2 of the accompanying drawings. In FIG. 2, T represents the period of a frame, SYNC represents the reference signal indicated by the broken-line arrows in FIG. 1, Va represents the A-roll video signal from the playback VTR 100, Vb represents the B-roll video signal from the playback VTR 101, and Vab represents the A/B-roll effect video signal from the switcher 102.
When the control signal from the controller 104 is supplied to the playback VTR 100 and the playback VTR 101, the playback VTR 100 and the playback VTR 101 start playing back their respective video tape cassettes in synchronism with the reference signal SYNC. As shown in FIG. 2, the playback VTR 100 outputs A-roll video signals Va1, Va2, . . . , Va6 successively in respective periods T1, T2, . . . , T6, and the playback VTR 101 outputs B-roll video signals Vb1, Vb2, . . . , Vb6 successively in the respective periods T1, T2, . . . , T6.
These A- and B-roll video signals Va, Vb are supplied from the playback VTRs 100, 101 to the switcher 102. In response to the control signal from the controller 104, the switcher 102 selects and outputs the A-roll video signals Va1, Va2 in the respective periods T1, T2, outputs effect video signals Vab1, VaB2 produced by applying effects to the A-roll video signals Va3, Va4 and the B-roll video signals Vb3, Vb4 in the respective periods T3, T4, and selects and outputs the B-roll video signals Vb5, Vb6 in the respective periods T5, T6. The signals which are selected or given effects by the switcher 102 are outputted as the A/B-roll effect video signal Vab. The A/B-roll effect video signal Vab outputted from the switcher 102 is supplied to the recording VTR 103, which records the signal on the magnetic tape in the loaded video tape cassette along inclined tracks formed thereon.
Some A/B roll editing systems of the type described above have VTRs which employ magnetic tapes having widths of 1/2 inch, 3/4 inch, and 1 inch. In such A/B roll editing systems, a tracking servo CTL (control pulse) signal recorded in a control track on a magnetic tape by a dedicated head when a video signal is recorded is reproduced, pulses of the reproduced CTL signal are counted to detect the present position on the magnetic tape, and processing such as phase control or the like is carried out based on the detected positional information.
Other A/B roll editing systems of the type described above have VTRs which employ magnetic tapes having widths of 8 mm, and those VTRs do not have a tracking servo system using the CTL signal, but incorporate an ATF (Automatic Track Finding) track servo system. According to the ATF track servo system, there are three methods available for detecting the present position on the magnetic tape.
According to the first method, the present position on the magnetic tape is determined by counting valleys or peaks of the envelope of a reproduced video signal (RF signal) supplied from a playback head.
However, the first method is disadvantageous in that if the envelope of a reproduced video signal is not obtained due to a dropout of the reproduced video signal or the like, the count of envelope valleys or peaks will be inaccurate, failing to produce accurate tape position information.
According to the second method, a time code representing tape position information is recorded in a PCMID area as a user area or a coding index area of PCM audio data recorded on a magnetic tape according to the 8-mm video tape format, and, when the magnetic tape is played back, the time code is read, and the tape position information is produced based on the time code that has been read.
The second method requires for its implementation a time code generator and a time code reader which add to the cost of the editing system. In addition, when the VTR operates in a variable-speed playback mode such as a search mode, since the head does not accurately scan a track on the magnetic tape where a PCM signal is recorded, e.g., obliquely scans two tracks on the magnetic tape, errors may be caused in reading the recorded time code, making it impossible to detect the present position on the magnetic tape with accuracy.
According to the third method, a capstan speed signal indicative of the detected rotational speed of a capstan motor or a reel speed signal indicative of the detected rotational speed of a reel motor is frequency-divided, and pulses of the frequency-divided signal are counted to detect the present position on the magnetic tape.
With the third method, inasmuch as the present position on the magnetic tape is not detected based on the video signal recorded on the magnetic tape, if the capstan slips against the magnetic tape or the magnetic tape is elongated or contracted due to aging or temperature changes, then the reproduced video signal tends to be brought out of phase with the frequency-divided signal produced from the capstan speed signal or the reel speed signal, resulting in a failure to detect the present position on the magnetic tape with accuracy.
The applicants have proposed a video signal reproducing apparatus which corrects the phase of pulses produced by frequency-dividing a capstan speed signal based on tape speed information that is obtained from an RF output signal and a switching pulse signal and phase information about the RF output signal and the switching pulse signal, for thereby generating a quasi-CTL signal which represents the accurate present position on the magnetic tape (see Japanese laid-open patent publication No. 2-292770).
In a VTR which uses a signal format that does not record a CTL signal on a magnetic tape, the proposed video signal reproducing apparatus corrects the phase of pulses produced by frequency-dividing a capstan speed signal based on tape speed information that is obtained from an RF output signal and a switching pulse signal and phase information about the RF output signal and the switching pulse signal, for thereby generating a quasi-CTL signal which represents the accurate present position on the magnetic tape. The quasi-CTL signal thus generated makes it possible to position the magnetic tape accurately for thereby effecting complicated tape editing.
The quasi-CTL signal generated in the manner described above is highly accurate. However, the RF signal which is employed in the generation of the quasi-CTL signal may suffer a low level of accuracy because of elongation or contraction of the magnetic tape due to aging or temperature changes, hitting engagement of a head with the magnetic tape at the time the head moves into abutment against the magnetic tape, the linearity of tracks on the magnetic tape, and guard band conditions. If the RF signal has a low level of accuracy, then the quasi-CTL signal is also low in its accuracy. Accordingly, tape position information or track position information which is obtained on the basis of the low-accuracy quasi-CTL signal is also low in its accuracy.
For example, if the playback VTR 100 fails to be locked in phase with the reference signal SYNC in an editing process on the A/B roll editing system shown in FIG. 1, then the A-roll video signal Va from the playback VTR 100 is out of phase with the reference signal SYNC, as shown in FIG. 3 of the accompanying drawings. As a result, the A-roll video signal Va2, immediately prior to the application of effects, of the A-roll video signal Va outputted from the switcher 102 is subject to a dropout.
For the above reasons, an editing process that employs a VTR which produces a quasi-CTL signal is liable to develop an asynchronous condition upon switching between a video signal from an external source and a video signal generated by the VTR, and is difficult to synchronize the tape transport in the VTR with an external synchronizing signal for outputting a reproduced signal accurately at intended times.
Problems with an editing process which is carried out in synchronism with an external synchronizing signal in a VTR which has no CTL head will be described below with reference to FIG. 4 of the accompanying drawings.
FIG. 4 shows the relationship between the timing of frame pulses Fp and drum switching pulses SWp and the processing timing of a playback system of a VTR which divides one frame of a video signal into ten tracks and records them on a magnetic tape.
The timing of frame pulses Fp and drum switching pulses SWp and the processing timing are illustrated in an upper portion of FIG. 4, and blocks of the playback system are illustrated in a lower portion of FIG. 4. The playback system includes a demodulator 43a, a time base corrector (TBC) 43b, an error correcting circuit 43c, a video decoder 43d, an audio decoder 43g, and D/A converters 43e, 43h.
In FIG. 4, L1, L2, . . . , L5 represent lock positions, de1 represents a processing time from the time when data recorded on a magnetic tape 28 by an A head 26p or a B head 27p is reproduced until the reproduced time is demodulated by the demodulator 43a and supplied to the TBC 43b, de2 represents a processing time from the time when the reproduced data is supplied to the TBC 43b until it is outputted from the TBC 43b, and de3 represents a processing time from the time when the reproduced data is outputted from the TBC 43b, and then processed by the error correcting circuit 43c, decoded by the video and audio decoders 43d, 43g, converted into analog video and audio signals by the D/A converters 43e, 43h, until the analog video and audio signals are outputted from respective output terminals 44, 45. The video signal outputted from the output terminal 44 is represented by Pout. In the example shown in FIG. 4, as can be seen from the solid-line arrow which indicates the video signal Pout from the time of the lock position L4, the video signal reproduced by the A head 26p or the B head 27p at the time of the lock position L4 is outputted at the position of a frame pulse Fp at a time T0 (reference playback phase).
The processing times (delay times) de1, de3 are fixed, and the processing time (delay time) de2 is variable.
In order for a desired video signal (and also the audio signal) Pout to be outputted at the time T0, the following equation has to be satisfied: EQU The time at which the playback head traces the position on the magnetic tape where the desired video signal is recorded=the time T0-the processing time de1-the processing time de2-the processing time de3. (1)
From the equation (1) can be derived the following equation (2): EQU The time at which the playback head traces the position on the magnetic tape where the desired video signal is recorded+processing time de2=the time T0-the processing time de1-the processing time de3. (2)
Therefore, if the lock position of the A head 26p or the B head 27p is changed, then in order for the video signal to be outputted at the time T0, the variable processing time de2 is adjusted because the processing times de1, de3 are fixed.
For example, if the A head 26p or the B head 27p is locked in the lock position L1, then the TBC 43b keeps a processing time de2 corresponding to three tracks, i.e., three switching pulses SWp, for outputting the reproduced data at the time T0. The three tracks referred to above mean three tracks for the A head 26p and three tracks for the B head 27p. If, however, the TBC 43b keeps a processing time de2 corresponding to two tracks, then the reproduced data is outputted one track prior to the time T0.
If the A head 26p or the B head 27p is locked in the lock position L5, then the TBC 43b is required to maintain the processing time de2 as it is or adjust it for one period. As a consequence, the reproduced data of a preceding frame is outputted at the time T0.